When starting up or accelerating, the force transmission depends on the slip between tire and carriageway. This dependence can be represented in adherence-slip curves. In the case of small slip values, the acceleration processes proceed in the stable region, while if the slip increases there is also an increase in the useful adherence, that is to say the coefficient of static friction. With increasing slip beyond the achievable maximum adherence, the unstable region of the slip curve is reached because further increase in the slip leads to a reduction in the adherence.
In order to adapt the traction slip to permissible values, use is made of traction slip regulators (ASR) that, firstly, regulate the drive torque of the drive engine and, in addition, activate the brake system of the vehicle. Electronic brake systems (EBSs) of modern utility vehicles, in which ABS systems are also integrated, therefore have brake control units with a functional part for regulating traction slip.
FIG. 1 shows schematically the design of a control loop 10 for controlling the drive torque of a drive engine 12 of a utility vehicle. In this case, wheel rotational speed sensors are used to measure the wheel rotational speeds of the driven wheels, present at an end of a drive train 14, and corresponding signals are fed into a brake control unit 16 with integrated functional part for the purpose of traction slip control ASR. This brake control unit 16 is, by way of example, connected via a CAN interface 18 according to SAE J1939 to an engine control unit 20 that activates the drive engine 12. Via the interface 18, it is then possible for the brake control unit 16 to supply a desired value for the drive torque of the drive engine 12 to the engine control unit 20.
FIG. 2 shows the design principle of the control structure of the engine torque control within the traction slip control. In this case, a desired value is firstly calculated for the wheel speed from the instantaneous wheel speed of a driven wheel, the instantaneous vehicle speed, and a permissible traction slip. This desired value is compared with the actual value of the instantaneous wheel speed, and the speed control deviation is determined. The desired value for the drive torque of the drive engine 12 is calculated by the brake control unit 16 with the aid of an electronic controller as a function of this speed control deviation. It is therefore an essential feature of this control that the manipulated variable calculated by the controller is a desired drive torque for the drive engine 12, whereas the wheel speeds of the driven wheels are used as input variables. However, since there are further, subordinate control loops that influence the drive torque of the drive engine 12, this prescription of desired values is often not implemented identically in the engine control unit 20.
The German patent document DE 198 37 521 A1 discusses a method and a device for traction slip control in the case of which a switchover is made to engine rotational speed desired control in the run-up of a traction control. However, the document includes no accurate details on how such a control is to be implemented.
The aim of the method in WO 01/28802 is the avoidance of damage to differential gears or tires owing to excessively large differences in rotational speed between the driven wheels of an axle. By contrast with traction slip control on which the exemplary embodiments and/or exemplary methods of the present invention is based, a substantial difference resides in the fact that the aim of the known method is to limit the relative traction slip (wheel speed difference between the wheels of the front axle and the rear axle) between driven wheels of a vehicle to a prescribed value while, however, in the case of traction slip controls the absolute traction slip of the driven wheels, that is to say the slip between wheel and street, is controlled to an optimum value in order to be able to start up or accelerate with the best adherence between wheel and carriageway.